The increase of CO2 in the atmosphere is thought to have a major effect on global climate. It is therefore desirable to reduce the emission of anthropogenic CO2 into the atmosphere. In addition to the development of low CO2 emission power plants, energy-saving automobiles and the increased use of renewable energy sources, the permanent storage of CO2 in subterranean geological formations can be an important means for reducing net CO2 emission.
An extensive review of existing CO2 Capture and Storage (CCS) projects and technology is given in the IPCC Special report on Carbon Dioxide Capture and Storage (Carbon Dioxide Capture and Storage, IPCC, 2005, editors: Metz et al., Cambridge University Press, UK; also available at: http://www.ipcc.ch). The paper SPE 127096 “An overview of active large-scale CO2 storage projects”, I. Wright et al. presented at the 2009 SPE International Conference on CO2 capture, Storage and Utilization held in San Diego, Calif., USA 2-4 Nov. 2009 provides a more recent update on existing large-scale CO2 storage projects. Of the commercial scale projects reviewed in these documents, the most significant in terms of cumulative volume injected are the Sleipner and In Salah projects.
The Sleipner CCS Project is located 250 km off the Norwegian coast and is operated by Statoil. It is a commercial scale project for the storage of CO2 in a subterranean aquifer in the Utsira formation at a depth of 800-1000 m below the sea surface. CO2 produced during natural gas processing is captured and subsequently injected underground into the brine-saturated unconsolidated sandstone formation. CO2 injection started in October 1996 and by 2008, more than ten million tons of CO2 had been injected at a rate of approximately 2700 tons per day. A shallow long-reach well is used to take the CO2 2.4 km away from the producing wells and platform area. The injection site is placed beneath a local dome of the top Utsira formation.
The In Salah CCS Project is an onshore project for the production of natural gas from a gas reservoir located in a subterranean aquifer. The aquifer is located in the Sahara desert. The reservoir is in a carboniferous sandstone formation, 2000 m deep; it is only 20 m thick, and of low permeability. Natural gas containing up to 10% of CO2 is produced. CO2 is separated, and subsequently re-injected into the water-filled parts of the reservoir.
A known problem of CO2 sequestration in aquifers, particularly saline aquifers is the risk of salt precipitation, which can impair the injection of CO2. Salts are normally dissolved in formation water and can precipitate and form solids under certain conditions. When dry liquid or supercritical CO2, also known as “dense state” CO2, is injected into such formations, the water in the brine dissolves in the CO2. As water is removed into the CO2 stream, salt concentration increases, eventually reaching the solubility limit and giving rise to salt precipitation. The precipitated solids reduce the pore space available to the fluids, in some cases blocking the pore throats in the sedimentary rock. This impairs permeability near the wellbore, preventing fluid movement through the pores and may hinder any further injection of CO2. This phenomenon occurs at the CO2 injection points in and close to the borehole.
The book “CO2 Capture Project, a technical basis for carbon dioxide storage” edited by Cal Cooper, ISBN 978-1-872691-48-0, suggests injecting fresh water prior to the CO2 injection, in order to flush brine from the injection point. A further proposal is to use high injection rates in order to overcome the capillary forces with high fluid pressures. This latter proposal is limited by the supply of CO2, the surface facility specifications and, of course, the fracture gradient of the cap rock.
The paper “Optimization of Residual Gas and Solubility Trapping for CO2 Storage in Saline Aquifers” by Long Nghiem et al. presented at the 2009 Society of Petroleum Engineers Reservoir Simulation Symposium in Texas, USA, 2-4 Feb. 2009 proposes the use of a water injector above the CO2 injector to accelerate and increase residual gas an solubility trapping in low-permeability aquifers. The water flows downwards and meets the CO2, which flows upwards in the reservoir. The quantities of water required are considerable.
Two further publications, JP 3258340 A and WO 08/058298 propose the dissolution of CO2 in water to generate carbonated water prior to its injection into a subterranean reservoir. In both cases, the quantities of water required are substantial.
In view of the above described state of the art it is an object of the present invention to provide an alternative method and arrangement for the permanent storage of CO2 in aquifers where the risk of salt precipitation when injecting substantially pure CO2 is high.
It is a further object of the present invention to provide a method and arrangement which allow for a more efficient use of the storage capacity of aquifers for permanent storage of CO2.